1. Field of the invention
This invention relates to a thermal line printer in which adequate energy can be applied to a thermal head and in which transfer of print data to the thermal head can be effected at high speed.
2. Description of the prior art
Recently, thermal line printers have always been required to have high-speed and high-quality printing capabilities. Conventional thermal line printers will be described below.
As a process for control of energy to be applied to a line-type thermal head, a method is well known in which a circuit shown in FIG. 1A is utilized to control the energy to be applied to the thermal head by the use of a thermal time constant of heat generating elements of the thermal head. The operation of the circuit shown in FIG. 1A will be explained with reference to the timing chart of FIG. 1B. First, a head development signal (IN) is integrated by means of an RC circuit as indicated by the wave form P2 of FIG. 1B. The integrated wave form and the reference voltage V.sub.ref are compared with each other by means of a comparator 101. As can be seen from FIG. 1B, while the voltage of the signal P2 is greater than a predetermined voltage, the head development signal IN is suppressed at an AND gate 102 by the output from an inverter 103, resulting in a short period of "high level" for the output signal OUT which corresponds to the head development signal IN. Another process is such that corrective print data is transferred to the thermal head according to calculations within a microcomputer or such that a large number of shift registers are used to form a corrective print data generating circuit.
However, with the above-mentioned conventional arrangements, and particularly with the technique for controlling heat accumulation in the thermal head by utilizing the arrangement shown in FIG. 1, although application of pulses to the thermal head for one dot line at a time may eliminate the effect of heat accumulation when viewed in dot line terms, individual heat generating elements are not free from the effect of heat accumulation. Therefore, dots differing in size are printed with the result of considerable deterioration of print quality.
The arrangement for corrective print data transfer according to calculations within the microcomputer is disadvantageous in that since corrective print data is calculated on a soft basis, a considerably long time is required for data processing, which retards high speed printing. The arrangement utilizing a large number of shift registers is disadvantageous in that it involves a very high cost in providing of a control circuit and in that it requires a large space for installing shift registers. Furthermore, it requires considerable expenditure for gate arrays.